410 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1962 



genetic constitution, it is not easy to estimate the proportion of favor- 

 able to unfavorable mutations. Theoretical considerations and a cer- 

 tain amount of experimental evidence agree in indicating that the 

 great majority are unfavorable. Organisms are in general already so 

 highly selected for success in their normal environments that the 

 chance of further improvement by random mutation must be very 

 small. Perhaps an analogy with a fine watch will dramatize the 

 point. Assume the watch is very slightly out of adjustment. A ran- 

 dom change brought about, say by dropping it, could conceivably im- 

 prove the adjustment. Clearly, however, the chance of making it run 

 less well or not at all is enormously greater. Now let us extend our 

 typing analogy. Assume our inspector exercises judgment. Wlien 

 the typist makes an error that improves the original message, he 

 passes it. Thus improved messages will replace their ancestral forms 

 and the improvement will be cumulative. Something like this 

 happens with living systems. Specifications improved by occasional 

 favorable mutations are preferentially reproduced and thus tend to 

 replace their ancestral forms. This is natural selection. 



In recent years many factors have been found to increase the fre- 

 quency of mutations. High energy radiation that penetrates to the 

 cell nucleus is mutagenic in proportion to its amount. A number of 

 chemical agents are likewise mutagenic. It is now possible, for ex- 

 ample, to alter nucleotides in known chemical ways that will produce 

 mutations. Oxidation of amino groups of nucleotides with nitrous 

 acid is one way. It is encouraging that biochemists and geneticists 

 who study the mechanisms involved are beginning to be able to pre- 

 dict successfully the types of mutations that are most likely to be 

 produced by specific chemical agents. It is not, however, possible to 

 do this specifically for certain genes only. 



Let us now turn to the general question of evolution. Wliat do 

 mutations have to do with the processes by which evolution occurs? 

 It is especially appropriate at this time to discuss this aspect of my 

 subject, for, as you know, this is the hundredth anniversary of the 

 publication of Darwin's "Origin of Species." 



Organic evolution is interesting and important in many respects. 

 For one thing, it is not logically possible to accept only a small amount 

 of it, for one cannot imagine a living system that could not have 

 evolved from a very slightly simpler system. Starting with man, 

 for example, and working backward toward simpler systems one sees 

 no obvious stopping place. Our ancestors were presumably a bit 

 simpler than we. Early in man's evolution there were primitive men. 

 And before primitive man there were prehuman ancestral forms capa- 

 ble of evolving into true man. This is true however one defines man. 

 The point is that no matter what living system one thinks of, another 

 is conceivable that is one mutation simpler or different. And so one 



